Examples of methods of making yttrium tantalate x-ray phosphors are taught in U.S. Pat. Nos. 4,970,024, 5,009,807, 5,112,524 and 5,141,673. In particular, U.S. Pat. No. 5,112,524 teaches that small additions of aluminum, strontium and rubidium, between 0.001 and 0.1 moles per mole of yttrium tantalate, increase the brightness of unactivated yttrium tantalate x-ray phosphors. However, to obtain higher brightness, it has been necessary to use a lithium chloride containing flux to form the unactivated yttrium tantalate phosphor containing the aluminum additive. In particular, the prior art teaches that the highest brightness is obtained when a lithium chloride-lithium sulfate, (LiCl).sub.2 -Li.sub.2 SO.sub.4, eutectic flux is used. However, because of its corrosiveness and volatility during firing, the lithium chloride in the flux causes significant damage to the alumina firing crucibles and the refractory brick lining in the firing furnace. Additionally, the lithium chloride is the more expensive component of the (LiCl).sub.2 -Li.sub.2 SO.sub.4 eutectic flux and its presence in the flux makes it more difficult and less economical to reclaim the flux from the process waste streams. Thus, it would be a great advantage to be able to produce an unactivated yttrium tantalate phosphor containing an aluminum additive and having a brightness equivalent to or greater than those produced by the prior art methods without having to use a lithium chloride containing flux.